1. Field of the Invention
The present invention relates to a method and a system for generating switching control signal separating a transmission signal in an optical repeater employing a Time Division Duplex (TDD) scheme.
2. Description of the Related Art
Various wireless communication services using a wireless network have been created resulting from the rapid development in computers, electronics, and communication technology. The most common wireless communication service is a wireless voice call service for providing a user of a mobile communication terminal with the voice call wirelessly, which can provide the service anywhere at anytime. Further, the service completes the voice call service by providing a text message service. Recently, a wireless internet service has been on the rise which provides the user of the mobile communication terminal with an internet communication service through a wireless communication network.
The International Mobile Telecommunication 2000 (IMT-2000) of the third generation (3G) mobile communication system has been adopted as a standard by an International Telecommunication Union Recommendations (ITU-R), such as a Code Division Multiple Access (CDMA) system, an EV-DO, a Wideband CDMA (WCDMA) system. The IMT-2000 is the mobile communication system developed aiming at (1) implementing a worldwide direct roaming service including the personal and service mobility, (2) having speech quality equal to that of a wire telephone, (3) having various application services by combining high speed packet data services and a wire/wireless network, which enables to improve the existing quality of speech and a Wireless Application Protocol (WAP) service and (4) providing various multimedia services such as Audio On Demand (AOD), Video On Demand (VOD) or the like with a faster speed as well.
However, there is a limit in providing a very high-speed wireless internet in the existing mobile communication system because (1) the cost for constructing a base station is high, (2) the service charges of the wireless internet are expensive, and (3) usable content is limited due to a small size of a display unit of the mobile communication terminal or the like. Also a Wireless local Area Network (WLAN) scheme is limited in providing public services due to problems of wave interference and a narrow coverage area or the like. Therefore, a Wireless Broadband Internet (WiBro) of the very high-speed portable internet service and the fourth generation (4G) wireless mobile communication which can guarantee the portability and mobility and use the very high-speed wireless internet service at a lower cost has emerged.
The WiBro and the 4G wireless mobile communication employ a portable internet technology of a Time Division Duplex (TDD) scheme included in duplex schemes rather than in the CDMA and WCDMA. Also, an Orthogonal Frequency Division Multiplexing (OFDM) scheme is used for a modulation system.
Here, the TDD scheme is based on a bidirectional transmission scheme that an uplink and a downlink is divided on a time basis by turns in an identical frequency bandwidth. The TDD scheme has a higher transmission efficiency than a Frequency Division Duplex (FDD) scheme for allocating two different frequencies at an uplink and a downlink at different frequencies. The TDD scheme is suitable for transmitting asymmetric or bursting applications due to a dynamic allocation of a time slot. The OFDMA/TDMA schemes are a multiple access scheme similar to the TDMA scheme which allocates all sub-carriers spread over entire bandwidths to a user for a given time and to another user for the rest of the time. The OFDMA/TDMA schemes improve transmission speed per a bandwidth and prevent multipath interference.
The general mobile communication system, a mobile communication service region is divided into a plurality of cells using a frequency reuse concept or the like in order to expand the coverage of the mobile communication network. A wireless base station is constructed around centers of the respective cells in order to process the mobile communication service. Here, a radius of the cell is determined according to the strength of the signal of a corresponding region or an amount of data traffic. That is, the radius of the cell is set to be small in the city having lots of the data traffic amount and large in the city having relatively less amount of the data traffic. In doing so it makes the traffic generated from the respective cells not exceed the processing quantity of the wireless base station in charge of the corresponding mobile communication service.
There have been many efforts to provide better mobile communication service by appropriately adjusting the radius of the cell according to the frequency reuse concept, the amount of the traffic or the like. Nevertheless an electric wave shadow region where it is generally difficult to arrive the electric wave such as an underground, an inside of a building, a tunnel or the like still exists. To construct the plurality of new wireless base stations to solve the frequency shadow in the frequency shadow region causes the economical efficiency to be greatly decreased due to costs for facilities, construction, maintenance and repair or the like. Also it may bring an undesirable results in designing the cell. As a solution, the mobile communication service can be provided using an optical repeater system in the frequency shadow region. The optical repeater system makes a traffic channel allocated to a mother base station to be transmitted through an optical transmission scheme using the optical repeater to the electric wave shadow region thereby solving a problem of the electric wave shadow.
In particular, the 3G mobile communication system and the WiBro system use a higher frequency than the second generation (2G) mobile communication system and propagation path loss is large, diffraction effect is small and penetration loss of a building is large. Therefore, it is desirable to use the optical repeater having a small radius of the cell.
Further, a downlink signal and an uplink signal should be discriminated in order to relay a wireless signal between the base station and a terminal in the optical repeater. The FDD scheme used in the optical repeater of the mobile communication system discriminates the downlink signal and the uplink signal using the duplex. In contrast, the TDD scheme uses an identical frequency to transmit the downlink signal and the uplink signal and discriminates the downlink signal and the uplink signal by dividing time into a section so that it can not discriminate the downlink signal and the uplink signal by using the duplex. Therefore, the optical repeater using the TDD scheme can discriminate the downlink signal and the uplink signal by using a switch and selectively provide a path for the respective signal. To this end, a control signal is necessary so as to correctly identify a starting point of the downlink signal and a starting point of the uplink signal. In effect, switch on/off in response to the respective signals and change a movement path of the signal The optical repeater can receive the control signal from the base station through an optical cable.
The optical repeater employing the TDD scheme should analyze the transmission signal frame and generate switching control signal to control the switch to occur a switching operation between a downlink signal section and the uplink signal section. Meanwhile, the optical repeater transmits the signal through the optical communication cable so a time delay may occur during the transmission. If the switching control signal is not compensated for the time delay of the optical communication cable, an incorrect switching control signal may be generated. If the switching control signal is incorrect it is impossible to correctly discriminate between the downlink signal and the uplink signal.
A solution for the above problem is disclosed in the Korean First Patent Application No. 2006-0010963 entitled “Method and System for Generating Switching Timing Signal for Separating Transmitting and Receiving Signal in Optical Repeater of Mobile Telecommunication Network Using TDD and ODFM Modulation”.
FIG. 1 is a diagram illustrating an internal construction of a conventional TDD optical repeater.
FIG. 2 illustrates a structure of the uplink and downlink signal when data are transmitted in the conventional TDD optical repeater.
As shown in FIGS. 1 and 2, a main donor module 200 is connected to a base station 110 through an RF cable. If the main donor module 200 receives the RF signal from the base station 110, it converts the RF signal into an optical signal through an electrical-optical conversion. Then, it transmits the optical signal to a remote module 250 through an optical communication cable and converts the optical signal received from the remote module 250 into the RF signal through an optical-electrical conversion. The converted RF signal is transmitted to the base station 110 through the RF cable.
Further, if the remote module 250 receives the optical signal from the main donor module 200, it converts the received optical signal into the RF signal through the optical-electrical conversion. Then, the converted RF signal is transmitted to a terminal through an antenna, which converts the RF signal received from the terminal into the optical signal through the electrical-optical conversion. The converted optical signal is transmitted to the main donor module 200 through the optical communication cable.
A switching control signal generating circuit 290 detects a part of the RF signal from a coupler 265 and discriminates between a downlink signal and an uplink signal. A switching timing signal is generated capable of controlling a switch and transmits the switching timing signal to the switch 275. In order to generate the switching timing signal, the downlink signal transmitted from the main donor of the base station is transmitted included in control information. Then the switching timing signal for separating the uplink and downlink signal in a remote is generated.
When the downlink signal is input to the switch 275 by the received switching timing signal, the switch 275 radiates the downlink signal to the terminal through the antenna. When the uplink signal is input to the switch 275, the switch 275 interrupts a path connecting to a High Power Amplifier (HPA) 270 and sets the path for inputting the uplink signal to a Low Noise Amplifier (LNA) 280. That is, the terminal (MS) receives the signal directly received from the base station and the signal received passing through the donor 100 and the remote 300 as a multi-path signal. If the difference in the time delay between two signals exceeds a cyclic prefix time of an OFDMA symbol, an inter-symbol interference occurs between the two signals and the rate of data error is increased when the OFDMA symbol is demodulated. Also, it is difficult to adjust a time synchronization of a wireless signal transmitted/received in the base station and the remote 300 as well.